I guess it’s as a good a theory as any, but one would expect the theory to hold up against RHP too. That doesn’t seem to be the case. I think another plausible theory is that their LH batters (Abreu, Matsui, Damon, and Cano) all had prolonged slumps this year which made the lineup more vulnerable to LHP. When those guys are hitting well, the Yanks dominate LHP.